The invention relates to a drive arrangement for a planing boat which includes a motor with a drive shaft extending through the rear wall of the boat at an acute angle with respect to the water surface and carrying at its end a propeller which is arranged in a downwardly open, sidewardly pivotable, tunnel-like flow guide housing.
Such drives as shown for example in German Patent 3042197 have proved themselves quite well in practice since they permit highly efficient utilization of the given motor power and provide for excellent boat acceleration as well as high top speed.
It is the principal object of the present invention to improve the boat drive shown in German Patent 3042197, particularly to maximize the drive-influenced planing characteristics of a boat provided with such a drive.
During operation a planing boat by the design of its hull is subject to a dynamic-force generated lift which with increasing boat speed lifts the boat out of the water so that, at higher speeds, only the rear end of the boat is in contact with the water. This results in a substantially reduced resistance so that such boats can achieve relatively high speeds. It is noted that the reduction in resistance of planing boats is essential for reaching such high speeds. Consequently anything in the drive arrangement that may impair the resistance reduction or that may generate unnecessary resistance must be avoided. Particularly underwater components of the drive, if needed, must have a streamlined shape. For low resistance it is also advantageous if the hull bottom portions remaining in contact with the water are disposed at as small an angle as possible when the boat is gliding on the water surface. This angle however is determined to a large extent by the position and the kind of the boat drive. If as usual the drive is arranged low, that is, lower than the bottom of the boat, the drive force generates a large moment which tends to lift the bow and thereby prevents the boat from assuming its optimal planing angle with respect to the water surface. The boat planing angle can be corrected by trimming but only with some drive power losses. The same is true if the drive, that is, the propeller, is arranged at about the same level as the hull bottom as in the arrangement according to German Patent 3042197 but the drive shaft is arranged at an angle with respect to the hull bottom and the water tunnel extending to the rear and receiving the propeller is parallel to the drive shaft. In this case a negative moment is generated by the drive force which tends to push the bow down onto the water surface and this situation cannot be corrected by trimming of the outer drive structure. Since with such a direct shaft drive arrangement the angle of the drive shaft with respect to the water surface is inherently relatively large, that is, the propeller is not disposed in a plane normal to the water surface, the downwardly and upwardly moving propeller blades are at slightly different angles with respect to the water flow, that is, the downwardly moving blades are at a somewhat larger pitch than the upwardly moving blades which phenomenon reduces propeller efficiency and generates some high frequency vibration.
The best solution would be a drive which generates a drive force that is essentially in alignment with the hull bottom. This would avoid the need for trimming or other resistance inducing underwater components since all the resistance of a planing boat is generated at the hull bottom.